Mechanoreception

Mechanoreception, ability of an animal to detect and respond to certain kinds of stimuli—notably touch, sound, and changes in pressure or posture—in its environment. Sensitivity to mechanical stimuli is a common endowment among animals. In addition to mediating the sense of touch, mechanoreception is the function of a number of specialized sense organs, some found only in particular groups of animals. Thus, some mechanoreceptors act to inform the animal of changes in bodily posture, others help detect painful stimuli, and still others serve the sense of hearing.

Slight deformation of any mechanoreceptive nerve cell ending results in electrical changes, called receptor or generator potentials, at the outer surface of the cell, and this in turn induces the appearance of impulses (“spikes”) in the associated nerve fibre. Various laboratory devices are used to record and observe these electrical events in the study of mechanoreceptors. In addition to electrophysiological studies, mechanoreceptive functions are also investigated more indirectly—i.e., on the basis of behavioral responses to mechanical stimuli. These responses include bodily movements (e.g., locomotion), changes in respiration or heartbeat, glandular activity, skin colour changes, and (in the case of humans) verbal reports of mechanoreceptive sensations. The behavioral method sometimes is combined with partial or total surgical elimination of the sense organs involved. Not all the electrophysiologically effective mechanical stimuli evoke a behavioral response; the central nervous system (brain and spinal cord) acts to screen or to select nerve impulses from receptor neurons.

Humans experience pain as a result of stimulation of pain receptors (nociceptors), which are located in the skin and other tissues. Pain receptors respond to three different types of harmful (noxious, or nociceptive) stimuli: mechanical, thermal, and chemical. The pain sensation may be acute, involving a short-lived intense feeling of pain that subsides to dull throbbing, or chronic, involving long-lasting pain that often is associated with disease. The stimulation of pain receptors is characterized by a range of physiological and psychological responses, including an effort to withdraw from the stimulus. The reflex withdrawal of the hand from a flame, for example, may begin even before the person becomes conscious of the pain sensation.

Responses to painful stimuli also occur in nonhuman animals, and the question of how animals experience pain is of considerable interest to researchers. If a cat’s tail is accidentally stepped on, the animal’s cry and efforts to withdraw are so strikingly similar to human reactions that the observer is led to attribute the experience of pain to the animal. If one treads accidentally on an earthworm and observes the animal’s apparently desperate struggles to get free, the person might again be inclined to suppose that the worm feels pain. Whether that is the case is inherently uncertain.

The following observations illustrate some of the difficulties in making judgments of the inner experiences of creatures other than humans. After the spinal cord of a fish has been cut, the front part of the animal may respond to gentle touch with lively movements, whereas the trunk, the part behind the incision, remains motionless. A light touch to the back part elicits slight movements of the body or fins behind the cut. The head does not respond. A more intense (“painful”) stimulus (for instance, pinching of the tail fin), however, makes the trunk perform “agonized” contortions, whereas the front part again remains calm. To attribute pain sensation to the writhing (but neurally isolated) rear end of a fish would contradict evidence that persons with similarly severed spinal cords report absolutely no feeling (e.g., pain or pressure) below the point at which their cords were cut.

Aversive responses to noxious stimuli nevertheless have a major adaptive role in avoiding bodily injury. Without them, the animal may even become a predator against itself; bats and rats, for instance, chew on their own feet when their limbs are made insensitive by nerve cutting. Some insects normally show no signs of painful experience. A dragonfly, for example, may eat much of its own abdomen if its tail end is brought into the mouthparts. Removal of part of the abdomen of a honeybee does not stop the animal’s feeding. If the head of a blow fly (Phormia) is cut off, it nevertheless stretches its tubular feeding organ (proboscis) and begins to suck if its chemoreceptors (labellae) are brought in touch with a sugar solution; the ingested solution simply flows out at the severed neck.

At any rate, responsiveness to mechanical deformation is a basic property of living matter; even a one-celled organism such as an amoeba shows withdrawal responses to touch. The evolutionary course of mechanoreception in the development of such complex functions as gravity detection and sound-wave reception leaves much room for speculation and scholarly disagreement.

Reception of external mechanical stimuli

Sensitivity to direct tactual stimulation—i.e., to contact with relatively solid objects (tangoreception)—is found quite generally, from one-celled organisms up to and including humans. Usually the whole body surface is tangoreceptive, except for parts covered by thick, rigid shells (as in mollusks). Mechanical contact locally deforms the body surface; receptors typically are touch spots or free nerve endings within the skin, often associated with such specialized structures as tactile hairs. The skin area served by one nerve fibre (or sensory unit) is called a receptive field, although such fields overlap considerably. Particularly sensitive, exposed body parts are sometimes called organs of touch—e.g., the tentacles of the octopus, the beak of the sandpiper, the snout of the pig, or the human hand.

Stimulation of the human skin with a bristle reveals that touch (pressure) sensation is evoked only from certain spots. These pressure spots, especially those on hairless parts (e.g., the palm of the hand or the sole of the foot), are associated with specialized microscopic structures (corpuscles) in the skin. In humans, pressure spots are most densely concentrated on the face, lips, tongue, fingertips, palms, and soles of the feet. A characteristic feature of many tactile sense organs is their rapid and complete adaptation (i.e., temporary loss of sensitivity) when stimulated. Still, in humans a distinction can be made between transient and more prolonged pressure sensations.

In higher vertebrates, touch receptors known as Pacinian corpuscles occur under the skin, being abundant particularly around muscles and joints. Local pressure exerted at the surface or within the body causes deformation of parts of the corpuscle, a shift of chemical ions (e.g., sodium or potassium), and the appearance of a receptor potential at the nerve ending. This receptor potential, on reaching sufficient (threshold) strength, acts to generate a nerve impulse within the corpuscle. Among insects, movements of tactile hairs have been shown (sometimes specifically) to affect the receptor potential and the impulse frequency in the connected nerve fibre.

Many vertebrates and invertebrates can localize with some precision points of tactual stimulation at the body surface. Humans typically can still distinguish two sharpened pencil points, or similar pointed stimuli, when the points are separated by as little as about 1 mm (0.04 inch) at the tip of the tongue. (When moved closer together, the two points are perceived as one.) The human two-point threshold is about 2 mm at the fingertip, reaching 6 or 7 cm (2.4–2.8 inches) at the skin of the back. Such tactual ability serves blind people when they read raised type (Braille) with their fingers. Closely related functions include the ability to distinguish between tactile stimuli that differ qualitatively—for example, between a rough and a smooth surface. This ability is even observable in the ciliate Stylonychia (a one-celled relative of Paramecium).

Sensory contact with the ground below often informs animals about their spatial position. Nocturnal animals (for example, some eels) find shelter during the day by keeping as much of their skin as possible in contact with solid objects in the surroundings (thigmotaxis). Animals that live in running water usually maintain their position as they turn and swim head-on against the current (rheotaxis). Study of rheotaxic behaviour reveals that the sensory basis almost exclusively depends on visual or tactile stimuli (or both) arising from the animal’s movements relative to the solid bottom or surroundings. The long antennae of many arthropods (e.g., crayfish) and the lengthened tactile hairs (vibrissae) on the snouts of nocturnally active mammals (e.g., cats or rats) serve in tactually sensing objects in the vicinity of the animal’s body, extending and enriching the adaptive function of the sense of touch.

Mechanoreceptor function

All of the primarily aquatic vertebrates—cyclostomes (e.g., lampreys), fish, and amphibians—have in their outer skin (epidermis) special mechanoreceptors called lateral-line organs. These organs are sensitive to minute, local water displacements, particularly those produced by other animals moving in the water. In this way, approaching organisms are detected and localized nearby before actual bodily contact takes place. Thus, the lateral lines are said to function as receptors for touch at a distance, serving to perceive and locate prey, approaching enemies, or members of the animal’s own species (e.g., in sexual display behaviour).

Each epidermal lateral-line organ, called a sense-hillock or neuromast, consists of a cluster of pear-shaped sensory cells surrounded by long, slender supporting cells. The sense hairs on top of the sensory cells project into a jellylike substance (the cupula) that bends in response to water displacement. The cupula stands freely in the surrounding water, grows continuously (e.g., as a human fingernail), and wears away at the top. Sense organs of this type are distributed along definite lateral lines on the head and body of the animals, developing in the outer layer of cells (ectoderm) of the embryo from a thickening called the lateral placode. From the central part of the same placode the sensory cells of inner-ear structures (the labyrinth) arise. The common embryologic origin and structural similarities of mature neuromasts and labyrinthine cell groups have led to the designation of all of these organs as the acoustico-lateralis system. The nerves to all the sense organs of the system arise from a common neural centre (called the acoustic tubercle in the wall of the brain’s medulla oblongata). Among amphibians such as frogs, lateral-line organs and their neural connections disappear during the metamorphosis of tadpoles; as adults, they no longer need to feed underwater. The higher land-inhabiting vertebrates—reptiles, birds, and mammals—do not possess the lateral-line organs; only the deeply situated, labyrinthine sense organs persist.

The sensory cell of a neuromast bears one relatively long hair (kinocilium) and about 50 shorter ones (stereocilia). The kinocilium is inserted eccentrically on top of the sense cell; the stereocilia are arranged in parallel rows. In about half of the hair cells of a neuromast, the kinocilium is found on one (and the same) side of the cell; in the remaining hair cells it is found on the opposite side. In most cases these are cranial and caudal side, respectively. In the clawed frog (Xenopus), each group of hair cells in a neuromast connects to its own nerve fibre; hence there are two fibres per sense organ. The hair cells send a continuous series of neural impulses toward the acoustic tubercle in the absence of adequate external stimulation. A longitudinal water current along the toad’s body surface, however, selectively increases or decreases the frequency of impulses from the cranial and caudal cells, depending on whether the flow is from head to tail or vice versa; current directed at right angles to such neuromasts has no effect. The impact of the moving water moves the cupula to deform the sensory hairs. Even minute cupula displacements of less than one thousandth of a millimetre are clearly effective in altering the impulses.

In Xenopus, as well as in other animals that have lateral-line organs, there are also some neuromasts with their hair cells asymmetrical at right angles to the head-tail axis. These add directional sensitivity so that other animals moving nearby in the water are well distinguished and localized. The postulated function of the lateral-line organs in the reception of low-frequency propagated pressure waves (“subsonic sound”) has not been verified behaviorally. At very short distances, however, a vigorous low-frequency sound source stimulates the lateral-line system on the basis of acoustical near-field effects (water particle displacements), just as does any moving or approaching object.

Cyclostomes, many bony fishes, and all the aquatic amphibians studied have only superficial (“free”) neuromasts of the kind described above. In the development of most fish, however, a number of structures called lateral-line canals are formed as a secondary specialization. They begin as grooves that develop in the epidermis along the main lateral lines. Thus, a number of formerly free neuromasts are taken down to the bottom of each groove. The walls of the grooves then grow together above the neuromasts. Eventually the grown-together walls form canals under the epidermis, containing in their walls a series of canal neuromasts and a chain of openings to the outside (canal pores) along the lateral lines. The cupulae are changed in form, fitting the canal somewhat like swinging doors. The canal is filled with a watery fluid. Stimulation occurs essentially in the same way as with free neuromasts: local, external water displacement is transmitted via one or more canal pores to produce a local shift of the canal fluid to move cupulae. The sense cells in the canal neuromasts are polarized in the direction of the canal.

Canal specialization is particularly well developed in lively species of fish that swim more or less continuously and in bottom dwellers that live in running or tidal waters. Canalization has been interpreted as a case of adaptive evolution, serving to avoid the almost continuous, intense stimulation of free neuromasts by water flowing along the fish body during swimming or, in the case of relatively inactive bottom dwellers, by the external currents. These coarse water displacements probably mask subtly changing stimuli from detection by the lateral-line organs on the surface of the animal’s body. Canal neuromasts are shielded in large degree from these masking currents.

The lateral-line organs function mainly in locating nearby moving prey, predators, and sexual partners. Usually these objects must be much closer than one length of the animal’s body to be detected in this way; even intense stimuli are hardly ever detected beyond five body lengths away. Lateral-line function in rheotactic orientation against currents is restricted mainly to inhabitants of small currents, such as mountain brooks, where marked differences of water flow velocity affecting the fish body locally are likely to occur. Compared with their use of other sensory functions (e.g., vision), the animals depend little on ability to sense extremely close, resting objects (obstacles) through the lateral lines. Obstacle detection of this kind does not arise from reflection of water waves. Rather, the pattern of water displacement around the moving fish abruptly undergoes deformation at the near approach of an obstacle as the result of compression; the fish encounters a sudden rise in water resistance in the immediate vicinity of the obstruction. Nor do the lateral-line organs function to regulate or coordinate the animal’s movements on the basis of the water flow or pressure variations along its body produced by swimming. Neither do they serve for the reception of water-transmitted propagated sound waves (hearing).

A specialization of the lateral-line system is the formation in several groups of fish of deeply buried, single electrically sensitive organs. In the elasmobranchs (e.g., sharks and rays), such organs are found on the head and are called ampullae of Lorenzini. Similar organs include those on the head of eeltail catfish (Plotosus), a marine bony fish (teleost); the small pit organs of other catfish; structures called mormyromasts in freshwater African fish (mormyrids) and in electric eels (gymnotids); and possible related organs in several other fish groups. These various ampullary lateral-line organs have features in common. For example, the sensory cells are withdrawn from the body surface, lack kinocilia, and have no mechanical contact with the surrounding water through a cupula. The latter attribute, indeed, is typical for all the acousticolateral end organs, except ampullary sense organs, in which the sense cells lie within the wall of a vesicle (or ampulla) that opens to the surface through a tubelike duct. Ampulla and duct are filled with a gelatinous substance that has excellent electrical conductivity.

Fish with ampullary sense organs are found to be remarkably sensitive to electrical stimuli—i.e., minute, local potential differences in the surrounding water at their body surface. In behavioral experiments with sharks and rays, sensitivity to changes of 0.01 microvolt per centimetre (one microvolt = 1/1,000,000 of a volt) along the body surface has been found for the ampullae of Lorenzini. Similar, though somewhat higher, values have been recorded from the ampullary nerve fibres. A decrease in voltage at the opening of the ampulla causes an increase of the spontaneous nerve-impulse frequency; an increase in voltage at the opening produces the opposite response. Through their electrical sensitivity, such fish can detect and locate other organisms in darkness, in turbid water, or even when these organisms are hidden in the sand or in the mud of the bottom.

Sharks, rays, and most catfishes are able to detect electrical changes (biopotentials) emanating from other organisms. The freshwater mormyrids and eels, on the other hand, have special signal-emitting electric organs. They produce a series of weak electric shocks (up to a few volts), sometimes quite regularly and frequently—for example, about 300 shocks per second in the mormyrid fish Gymnarchus. In this way, a self-generated electric field is created in the immediate surroundings. Any appropriate object (for example, a prey animal with good conductivity in relation to fresh water) will cause a deformation of the electric field and can thus be detected in a radar-like manner through the sensitive ampullar electroreceptors.

Some theorists suggest that initially mechanoreceptive lateral-line organs evolved into electroreceptors. At any rate, evidence of a certain double sensitivity—to mechanical and to electrical stimuli—has been observed in electrophysiological experiments with Lorenzinian ampullae. This double sensitivity has not been found, however, in behavioral experiments; alterations in behaviour indicate that ampullary lateral-line organs merely serve the animal as electroreceptors in adapting to the environment.

Other varieties of mechanoreception

Surface waves

Several species of animals living at or near the water surface use surface waves or ripples emanating from potential or struggling victims to locate their prey quickly: examples are the clawed frog (Xenopus), several fish species, and insects such as the backswimmer (Notonecta) and the water strider (Gerris). The whirligig beetle (Gyrinus) also uses surface ripples to avoid collisions with obstacles and companions. The sensory structures involved range from specialized tactile hair receptors (trichobothria) to internally located cells (proprioceptors) in movable body appendages and lateral-line organs.

Water and air currents

Special water-displacement receptors found in lobsters (Homarus) are most reminiscent of the lateral-line organs in vertebrates. Water-current receptors also enable several kinds of bottom-dwelling invertebrates to orient themselves (rheotaxis) in rivers and tidal currents. Many predators among these animals also respond chemically, moving against the current (positive rheotaxis) until the prey is reached. In this way, for example, certain marine snails easily find their particular prey (sea anemones). Similarly among insects, the chemical “smell” of prey or of potential sex partners elicits a tendency to move against the wind (anemotaxis) until the source of the chemical stimulus is found. Several types of air-current receptors (true mechanoreceptors) on the heads of insects enhance such chemoreceptive behaviour. In flying locusts, an air current directed appropriately toward the head elicits compensatory reflex flight movements. The receptors involved (groups of hair sensilla on the head) mediate small corrections in the maintenance of straight flight; major guidance, however, derives from the insect’s visual contact with the ground below.

Adaptation and recovery occur most rapidly among touch receptors, and they tend to respond well to repeated stimulation, even of relatively high frequency. Thus, a person can feel whether an object is vibrating; above a threshold frequency of about 15 cycles per second (cps), discretely perceived tactual stimuli seem to fuse into a quite new and distinct vibratory sensation. The upper frequency limit of this vibration sense is found at several thousand cps among normal individuals, with sensitivity being maximal in the range of 200 cps (above a threshold amplitude of about 100 millimicrons). Just as pitch is discriminated in hearing, differences of about 12 to 15 percent in vibration frequencies can be distinguished by most people.

Vibration sensitivity is not limited to humans. Fish, for instance, also may respond to low-frequency water vibrations with tactile receptors. In addition, several kinds of animals have special vibration receptors. In some insects, a group of specialized structures (chordotonal sensilla) in the upper part of each tibial segment of the leg signal vibrations from the ground below. In the cockroach, the threshold amplitude for vibrational stimuli of this kind has been found to be less than 0.1 millimicron. Birds have special receptors (corpuscles of Herbst in the tibiotarsal bone of the leg) with which they can detect slight vibrations of the twig or branch on which they sit. Maximal sensitivity in birds is at about 800 cps, and the threshold amplitude is close to 20 millimicrons. Spiders also use their vibration sense to locate prey in the web.

Several types of aquatic animals are sensitive to small changes of hydrostatic, or water, pressure. Among fish, this applies particularly to the superorder Ostariophysi, which includes about 70 percent of all freshwater species of fishes. The swimbladder in these animals is connected with the labyrinth (sacculus) of the inner ear through a chain of movable tiny bones, or ossicles (weberian apparatus). Alterations in hydrostatic pressure change the volume of the swimbladder and thus stimulate the sacculus. These fish can easily be trained to respond selectively to minute increases or decreases in pressure (for example, to a few millimetres of water pressure), indicating that they have a most refined sense of water depth. Such fish are known as physostomes, which means that they have a swimbladder duct through which rapid gas exchange with the atmosphere can occur; many live in relatively shallow water. The hydrostatic pressure sense can function to inform the animals about their distance from the surface or about the direction and velocity of their vertical displacement. It also appears that improvement and refinement of the sense of hearing arises through the swimbladder’s connections via the weberian apparatus with the labyrinth.

The sensitivity of several kinds of crustaceans to relatively small hydrostatic pressure changes (as low as 5 to 10 cm [2 to 4 inches] of water pressure) is most remarkable because these animals have no gas-filled cavity whatsoever. The mechanism by which the stimuli are detected remains unclear, although information about changing water depth during tidal ebb and flow would seem to have adaptive value.